1. Field of the Invention
The present invention relates to a permanent-magnet magnetic field generator.
2. Description of the Related Art
A dipole-ring magnetic circuit comprises a plurality of magnet elements arranged in such a ring shape that the magnetization direction of the magnet elements can make a full rotation per half circle of the ring. The dipole-ring magnetic circuit is configured to generate a substantially unidirectional magnetic field in an internal space within the ring. Such a dipole-ring magnetic circuit is widely used, for example, for a magnetic resonance imaging (MRI) apparatus, a semiconductor device fabrication process, and a uniform magnetic field generator for fundamental research. Conventionally, normal electromagnets, superconducting electromagnets, or the like are used as a generator for generating a uniaxial uniform magnetic field. However, as a result of the recent development of high-performance rare earth permanent magnets, rare earth permanent magnets (hereinafter also referred to simply as “permanent magnets”) have become dominantly used for uniform magnetic field generators for weak magnetic fields of, for example, 2 T or less.
One of features of a dipole-ring magnetic field generator is a capability of forming a magnetic field, which is substantially unidirectional and has substantially uniform strength, in an internal space within the magnetic field generator. Here, consider the angle of magnetic field vector (hereinafter referred to as a “skew angle”) at each point in the internal space within the magnetic field generator, assuming that the direction of a NS magnetic field (Y-axis direction) generated in the space is 0°. Due to the characteristics of the magnetic field generator, the skew angle is substantially 0° near the central axis of the ring of the device, but tends to become worse, i.e., increase as the point becomes closer to the inner wall of the magnetic field generator. Thus, an effort has been made to reduce the skew angle (see Japanese Patent Application Unexamined Publication No. 2005-56903).